Atmosphere control in dip-forming process



n.1. mannen. Jn 3,484,280

mnosrnan cou'rnol. In DIP-Forums rnocnss rma prix 4. 196? 2 Sheets-$hoet1 /IV VENTO? ROLAND R CARREKA'@ ATTRN'E Y 2 Sheets-Sheet 2 med April 4,19?

1.o DussoLvEn HYoRoGEN' Ffm A TTR/VE Y United States Patent O 3,484,280ATMOSPHERE CONTROL IN DIP-FORMING PROCESS Roland P. 'Carrekerg Jr.,Schenectady, N.Y., assignor to General Electric Company, a corporationof New York Filed Apr. 4, 1967, Ser. No. 628,356 Int. Cl. C23c 1/10 U.S.Cl. 117--114 5 Claims ABSTRACT OF THE DISCLOSURE In the dip-formingprocess for accreting copper onto a core member passed upwardly througha crucible containing a molten bath of copper, a melt of molten copperis supplied to the crucible from a melting furnace. A reducingatmosphere comprising hydrogen, carbon monoxide, carbon dioxide, and aninert diluent in controlled amounts is maintained above the melt in themelting furnace, whereby the oxygen content in the melt is reduced toless than about parts per million.

The dip-forming process, such as used in casting metal, is now wellknown in the art. According to this process, an elongated body such as ametal rod is typically pretreated by passing it through a straightener,a surface cleaning apparatus, and then through a vacuum entrance chamberpositioned below or at the bottom of a crucible containing a bath ofmolten material. A continuous elongated core or body is passed upwardlythrough the molten bath, which bath may have the same chemicalcomposition as that of the core or a different composition. The moltenmaterial in the crucible accretes or deposits upon the outer surface ofthe core thereby increasing its crosssectional area appreciably. Afteremerging from the crucible, the resulting cast member is cooled as by awater spray from one or more jet nozzles at least sufficiently so thatthe cast member can be rolled or worked. From there, the cast member maybe passed through a suitable roll mechanism where it is reduced incross-sectional area and then passed to a suitable receiving means suchas a take-up reel.

The quality of the copper rod cast by the dip-forming process isinfluenced to a large extent by the gases dissolved in the moltencopper, which are dependent upon the composition of the atmosphere incontact with the molten metal. In casting copper, gas evolution resultsfrom the presence of excessive oxygen and/or hydrogen in the melt. Theproblem of gas contamination in the dipforming process has beenpreviously recognized, and in one aspect formed the basis of U.S. PatentNo. 3,060,053 to Carreker and Parke and assigned to the assignee of thisinvention.

Under the conditions employed in the dip-forming process, oxygen andhydrogen present in the atmosphere above the molten-metal can bedissolved by and/ or react with the motlen material. Oxygen may causethe formation of undesirable oxides. It is recognized that copper havingan oxygen content greater than approximately 20 parts per millionresults in progressive deterioration in casting quality. Also, thepresence of hydrogen alone, or hydrogen and oxygen concurrently, inmolten metal in sufficient amounts will cause formation of gas bubblesupon solidifcation of the accreted material on the cast member. Theformation of oxides, gas bubbles and/ or voids not only adverselyaffects the quality of the product, but also deleteriously affects theuniform accretion of molten material onto the core by creating regionsof relatively poor thermal conductivity within a mass of relatively goodthermal conductivity. Thus, the effect of oxygen and hydrogen is notconfined to the oxide phases and bubbles they create,

rice

but is magnified due to the irregularities in heat transfer theyproduce, causing substantial non-uniformity in the external contour ofthe accreted coating. Bubbles, voids, oxide phases, internaldiscontinuities, and irregular accretion are all undesirable in thatthey promote or cause the formation of cracks and fissures when stressesare imposed, as by bending, rolling, drawing or the like. A productmanifesting such defects is generally not acceptable.

According to U.S. Patent 3,060,053, referred to above, the oxygencontent in the molten metal is reduced to a very low amount by utilizinggraphite or comparable reducing means which will unite with the oxygenin the molten metal to form a gaseous product which escapes from themolten copper. In addition, an inert atmosphere such as nitrogen ismaintained above the surface of the melt to protect the melt from theair. Such a furnace system will produce satisfactorily low oxygencontent copper for use with dip-forming, if the furnace system is freeof substantial air leaks and is allowed to come to substantialequilibrium. However, air leaks may be expected in a large industrialfurnace, particularly under charging conditions, and long residencetimes for equilibration are not realized. Experience has demonstratedthat graphite furnace linings and graphite heating elements are rapidlyconsumed by oxygen from undesired air leaks. Ceramic furnace 1inings andsilicon carbide electrical resistance heating elements are not harmed bysuch air leaks. However, oxygen from the air may react with the moltencopper to form copper oxide slag, which will attack or erode the siliconcarbide furnace parts. Use of a reducing atmosphere, rather than aneutral or inert atmosphere such as nitrogen, is desirable. Hydrogen isa reducing gas, but, as explained above, this gas is readily absorbed bythe molten copper with can result in the formation. of gas bubbles,thereby causing porosity and interfering with uniform heat transfer.Carbon monoxide is another reducing gas, but it is insoluble in moltencopper and can react with the melt only at its surface. Therefore,carbon monoxide is relatively slow in reducing the oxygen content in arelatively deep melt. Further, strongly reducing atmospheres harmfullyaffect silicon carbide heating elements in that such atmospheres causethe formation of volatile silicon monoxide, which results in theeventual distintegration of the heating element.

The purpose of this invention is to provide a controlled reducingatmosphere above the molten copper which is sufficiently reducing tomaintain the oxygen in the molten copper at less than approximately 20parts per million, yet sufficiently oxidizing to avoid substantialerosion or attack of silicon carbide furnace elements employed in thesystem, and sufiiciently low in hydrogen to avoid evolution of hydrogenbubbles during solidification in the dip-forming process.

According to the present invention, molten copper from a melting furnaceis supplied to a crucible through which a continuous, elongated core rodis passed, whereby molten metal deposits or accretes to the rod. Areducing atmosphere comprising 1 to 10 percent by volume hydrogen, 1 to10 percent carbon monoxide, carbon dioxide, the ratio of carbon dioxideto carbon monoxide being from about 2:1 to 1:20, and the balance beingan inert diluent such as nitrogen is maintained above the molten metal.By reason of the reducing atmosphere maintained 3 tion between theatmosphere and the melt at low oxygen contents is reduced.

Reference is now had to the following detailed specification and to theaccompanying drawings illustrating a preferred embodiment of theinvention.

FIGURE 1 is a perspective view partially in section of an apparatus forpracticing the present invention.

FIGURE 2 is a graph showing the solubility of hydrogen in copper.

Referring to FIGURE 1, core rod is supplied to a drawing apparatus 12located adjacent a suitable shaving apparatus 14 which shaves a thinlayer of metal from the periphery of the rod to remove the oxide coatingand other surface contaminants therefrom. If desired, other cleaningmeans such as chemical cleaning means, may be utilized for removing theoxide coating and cleaning the surface of the rod. The rod is passedthrough a suitable drive means, including capstan 15 and drive rolls 16contained in housing 18, and from there into tube 20. Members 12, 14, 18and 20 are sealed and interconnected to a suitable passageway, and tube20 has connected thereto exhaust tube 22 and evacuating pump 24 tomaintain a vacuum in the passageway. Entry port member or nozzle 26 ismounted in the upper end of tube 20 and extends into crucible 28 whichis maintained at the desired elevated temperature by suitable heatingmeans such as electric induction heater 30. Core lrod 10 is fed throughnozzle 2-6 and into crucible 28 Where the rod comes into contact withthe molten copper which accretes to the rod, as described hereinafter ingreater detail.

Molten copper is supplied to crucible 28 from melting ifurnace indicatedgenerally by the numeral 32, which is heated by a plurality of siliconcarbide electric radiant heaters 34. Cathode copper plates 35, or othersuitable feed material of low oxygen content, is supplied to the furnace32 by any suitable means such as a mechanical drive means (not shown).Desirably the furnace is provided with a sloping or inclined section 36and an integral horizontal section 38. The cathode copper plates arecharged to section 36 of the furnace through a dapper-type door (notshown) and melted gradually on the incline as they pass to thehorizontal section 38 where a pool of molten metal collects. The moltenmetal flows by displacement over a Weir 40 into a level control section42 of the furnace. A suitable liquid level sensing control, indicatedgenerally by the reference numeral 44, which is com- -monly employed inthe art, controls subsurface flow through passageway 45 into dip-formingcrucible 28. An electric probe 46 in the crucible senses the bath leveland automatically regulates the level sensing control to maintain aconstant level in the crucible.

Copper supplied to the furnace may contain an undesirably high oxygencontent, or air may leak to the furnace such as through the entrancewayto the furnace through which the copper is charged. For this reason, itis necessary to maintain a controlled reducing atmosphere above themolten metal. The reducing atmosphere is supplied to the furnace throughline 48 from a suitable source (not shown). The reducingr atmospherelowers the oxygen content of the copper to less than 20 parts permillion and maintains this low level. As a result, the molten copper fedto the dip-forming crucible has this low oxygen content, and furtherdoes not contain sufficient hydrogen to cause hydrogen bubble formationduring soliditication. Desirably, an atmosphere of an inert gas such asnitrogen is maintained above the molten metal in the crucible which maybe supplied to the crucible through line 49 from source 50. As the corerod 10 passes through the molten bath of metal in the crucible, copperaccretes progressively thereon thereby forming a rod of increaseddiameter with molten metal substantially bonded to the core rod.

An elongated tube 52 extends upwardly from crucible 28, and theresulting cast rod having copper accreted thereto is passed out of thecrucible to the extension tube.

The cast rod emerging from the crucible and extension tube is at arelatively high temperature and is cooled somewhat by cooling means 54,such as a water spray nozzle, before the rod is passed through directionchanging rolls 56. From there the cast rod is directed to a suitablerolling mill and coiling apparatus (not shown) and to a storage area asdesired.

lIn practicing this invention, the reducing atmosphere supplied to thefurnace comprises about 1 to 10 percent by volume hydrogen, 1 to 10percent by volume carbon monoxide, carbon dioxide, the ratio of carbondioxide to carbon monoxide being from about 2:1 to 1:20, and the balancebeing nitrogen or another suitable inert diluent. As mentioned above,the reducing gases of the atmosphere, that is, the hydrogen and carbonmonoxide, react with the oxygen present in the molten metal therebyreducing the oxygen content to less than 2() parts per million. Theoxidizing tendency of the atmosphere at a given temperature is specifiedby the ratio of carbon dioxide to carbon monoxide or by the ratio ofwater vapor to hydrogen; specifying either one of these ratios specifiesthe other, at equilibrium, according to the chemical Equation 1Coz-l-HzCO +H2O 1 The oxygen content of the molten copper, atequilibrium is governed by Equation 2.

1/zOZO (in copper) (2) which is related, through Equation 2, to theCO2/CO ratio by Equations 3 and 4 and 5.

O (in copper) =K2[O2]1/2 (4) per million is related to the ratio of CO2to CO in the atmosphere by Equation 6.

[CO2]N [CO2] O KZKS [G01 N14 [C01 6) The constant K2K3 is approximately7, 14 and 16 at 1100", 1200, and 1300 C., respectively. Thus, as auseful first approximation, the oxygen content of the molten copper inparts per million may be said to be about ten times the ratio of percent carbon dioxide to percent carbon monoxide in the atmosphere, atequilibrium.

The presence of hydrogen offers more complete assurance againstoxidation. Hydrogen is soluble in molten copper, and the amountdissolved is related to the hydrogen content in the atmosphere above themolten metal and to the temperature. Solid copper at its melting pointof 1083o C. can retain up to 1.7 parts per million of hydrgen in solidsolution in equilibrium with a hydrogen pressure of 1.0 atmosphere, Ifthe melt contains more than 1.7 parts per million hydrogen, hydrogen gaswill be rejected as bubbles during cooling or solidication, therebycausing porosity in the solidified metal. In order to avoid theformation of hydrogen bubbles, the hydrogen content in the furnaceatmosphere above the melt is such that the molten metal can absorb nomore than the solid solubility limit of 1.7 parts per million. In thismanner there will be no rejection of hydrogen during solidification andtherefore no hydrogen bubble formation. In typical dip-formingoperations, the ambient temperature in the furnace may range from aboutthe melting point of -copper (l083 C.) to about 1300 C., and

more preferably from about 1125 C. to 1175 C. FIG- URE 2 illustratesgraphically the quantitative relationship between percent hydrogen inthe atmosphere and the amount which can be dissolved in liquid copper atseveral temperatures. In the graph, the dissolved hydrogen in parts permillion is plotted as the abscissa and the percent hydrogen in theatmosphere as the ordinate. From the graph, it can be seen that thepercent hydrogen in the atmosphere should not exceed about l percent byvolume at the melting temperature of 1083 C., or about 7 percent at 1200C., in order to avoid formation of hydrogen bubbles. In the preferredoperating conditions, the hydrogen content in the atmosphere above themelt should range from about 2 to 5 percent.

It will be observed that the combination of hydrogen and carbon monoxidein the atmosphere in controlled amounts offers the desired reduction inoxygen content and further avoids any adverse results where only one gasis used. Generally, the carbon monoxide content will range from about 1to l0 percent by volume, and a ratio of carbon dioxide to carbonmonoxide is from about 2:1 to 1:20. In the preferred embodiment thecarbon monoxide content is from about 4 to 8 percent 4by volume, and theratio of carbon dioxide to carbon monoxide is from about 1:2 to 1:10.However, carbon monoxide does not penetrate the melt and thereforereacts with the oxygen at or near the surface of the melt only. On theother hand, hydrogen dissolves readily in the molten copper and willreact with the oxygen deep in the bath. Also, carbon monoxide alone willattack the silicon carbide heating elements, and to avoid this acontrolled amount of carbon dioxide is included in the atmosphere. Forthese reasons, the ratio of carbon monoxide to carbon dioxide isparticularly important. If the ratio is too low (i.e., the carbondioxide content is high), the atmosphere will not be sutciently reducingto deoxidize the copper. If, on the other hand, the ratio is too high,the excessive reducing atmosphere will attack the silicon carbideheating elements.

To further illustrate the invention, copper rod was formed by passing acopper core rod having a diameter of 0.38 inch through a molten bath ofcopper contained in a crucible. The molten copper was supplied to thecrucible through an integrally connected spout extending from arefractory-lined melting furnace which was heated by a plurality ofsilicon carbide electric radiant heaters, as described above. A reducingatmosphere was maintained over the molten copper in the furnace whichcomprised 5 percent hydrogen, 7 percent carbon monoxide, 0.7 percentcarbon dioxide and the balance nitrogen, all percentages being byvolume. Also, a protective atmosphere of nitrogen was maintained abovethe molten copper in the crucible. The resulting cast rod exhibited highquality being substantially free from irregularities and contamination,and upon analysis showed an oxygen content of about 5 parts per million.In a second run, the reducing atmosphere maintained above the moltencopper in the furnace comprised 13 percent hydrogen, 10 percent carbonmonoxide, 1 percent carbon dioxide and the balance nitrogen, allpercentages being by volume. The resulting cast rod from this second runhad a low oxygen content of 3 parts per million, but nevertheless was apoor quality exhibiting lhigh porosity resulting from hydrogenevolution.

I claim:

1. In a method of accreting copper onto a core rod comprising supplyingto a crucible a melt of molten copper from a melting furnace, passingsaid core rod through the molten copper contained in said crucible toaccrete copper thereon and removing the resulting cast rod from saidcrucible, the improvement comprising: maintaining above said melt in themelting furnace an atmosphere comprising from about 1 to 10 percent byvolume hydrogen, about 1 to 10 percent by volume carbon monoxide, carbondioxide, the ratio of carbon dioxide to carbon monoxide being from about2:1 to 1:20, and the balance being an inert diluent, whereby the oxygencontent in said melt is maintained at less than approximately 20 partsper million.

2. A method according to claim 1 wherein the hydrogen content is fromabout 2 to 5 percent.

3. A method according to claim 1 wherein the carbon monoxide content isfrom about 4 to 81 percent.

4. A method according to claim 1 wherein the ratio of carbon dioxide tocarbon monoxide is from about 1:2 to 1:10.

5. In a method of accreting copper onto a core -rod comprising supplyingto a crucible a melt of molten copper from a melting furnace, passingsaid core rod through the molten copper contained in said crucible toaccrete copper thereon and removing the resulting larger cast rod fromsaid crucible, the improvement comprising: maintaining above said meltin the melting furnace an atmosphere comprising from about 2 to 5percent by volume hydrogen, about 4 to 8 percent by volume carbonmonoxide, carbon dioxide, the ratio of carbon dioxide to carbon monoxidebeing from about 1:2 to 10:1, and the balance being nitrogen, wherebythe oxygen content in said melt is maintained at less than approximately20 parts per million.

References Cited UNITED STATES PATENTS 1,903,497 4/ 1933 Alexander etal. 75-76 1,958,754 5/1934 Holley 75--76 X 2,060,073 11/ 1936 Heuer75-76 X 2,060,133 11/1936 Summey 75--76 X 2,060,137 11/1936 Bahney 75-762,166,354 7/1939 Heuer 75-76 2,190,570 2/ 1940 Orlando 75-76 2,216,51910/1940 Quarnstrom 117--114 X 3,235,960 2/1966 Carreker 117-128 XFOREIGN PATENTS 499,728 2/ 1954 Canada.

ALFRED L. LEAVITT, Primary Examiner I. R. BATTEN, JR., AssistantExaminer' U.S. Cl. X.R.

